a) Field of the Invention
The present invention relates to a circuit for applying a direct current to a winding of a transformer, a choke coil or the like used in a variety of transmission circuits.
b) Description of the Related Art
In a conventional facsimile apparatus, for example, in order to pick up an alternating current (AC) component from a signal containing a direct current (DC) component and the AC component, a transformer shown in FIG. 4 is used. In this drawing, a primary winding of a transformer T.sub.1 and a choke coil L.sub.1 are connected in parallel with each other via a DC-isolation capacitor C.sub.1 and a secondary winding of the transformer T.sub.1 is coupled with a load impedance Z. Hence, when an AC/DC superimposed signal is input to the circuit of the primary winding side of the transformer T.sub.1, only the AC component is applied to the transformer T.sub.1. However, in this construction, the choke coil L.sub.1 becomes large and as a result, the size of this entire circuit also becomes large. Further, since the choke coil L.sub.1 is used, the cost of the parts become expensive.
In order to solve such a problem, for instance, a DC-superimposable transformer T.sub.2 can be used without requiring the choke coil L.sub.1 and the DC-isolation capacitor C.sub.1, as shown in FIG. 5. In this DC-superimposable transformer T.sub.2, a large gap is provided in its magnetic path so as to negate the influence of the DC-magnetization, permitting the applying of the DC component.
However, in the construction shown in FIG. 5, since the gap is provided so as to kill the influence of the DC-magnetization, the external shape and size of a core of the transformer T.sub.2 becomes large and thus its overall dimension becomes large. For example, in the use in the aforementioned facsimile apparatus, numbers of turns of the primary and secondary windings of the trans-former T.sub.2 are each 1600 and its core size is 35 mm (a so-called EI-35). Furthermore, in this case, even when such a large sized transformer T.sub.2 is used, large attenuation of approximately 3 dB at 100 Hz is results, as shown in FIG. 6.